1. Field of the Invention
The present invention relates to a system for and a method of measuring a position of a moving body for use in a car navigation system, a car locator or the like.
2. Description of the Prior Art
Recently, the car navigation system using a position receiver in the form of a receiver in a satellite navigation system, such as, the Global Positioning System (hereinafter referred to as "GPS") has been attracting attention in the field of car electronics.
Hereinbelow, a car navigation system using a conventional GPS receiver as disclosed in, such as. Japanese First (unexamined) Patent Publication No. 4-265879 will be described with reference to FIGS. 54 and 55.
In FIG. 54, the car navigation system includes a GPS receiver 100 which receives and demodulates GPS signals indicative of time data sent from a plurality of GPS satellites on radio waves, a CD-ROM 101 which stores map data, a display unit 102 and a CPU 103 which derives a car position based on the demodulated GPS signals and reads out the corresponding map data from the CD-ROM 101 for displaying it with the derived car position on the display unit 102.
An operation of the car navigation system as structured above will be described using a flowchart of FIG. 55 which is executed by the CPU 103.
At step 300, the CPU 103 reads the received GPS data demodulated at the GPS receiver 100. Subsequently, step 301 determines whether the GPS data from at least three GPS satellites are available for determining a car position. When answer at step 301 is positive, then the routine proceeds to step 303 where a current car position is derived based on the GPS data read out at step 300. On the other hand, when answer at step 301 is negative, the routine proceeds to step 302 where the GPS data read out at step 300 are corrected based on the stored GPS data for the corresponding satellites. The stored GPS data are the newest data used at step 303 in a previous execution cycle of this routine when step 301 yielded the positive answer. Subsequently, the routine proceeds to step 303 where a current car position is derived based on the GPS data corrected at step 302. Thereafter, step 304 reads out the corresponding map data from the CD-ROM 101, and step 305 processes the map data and the derived car position so as to be outputted at step 306 to the display unit 102 where the derived car position is displayed on the map data.
The foregoing conventional car navigation System, however, has the following disadvantage:
When the GPS data are not sufficient for determining the car position as determined at step 301, the GPS data are corrected using the stored previous data so as to derive the car position. Although the corrected GPS data are estimated or assumed data derived based on the stored previous data, the car position derived based on those estimated data is directly displayed for a user. Accordingly, the user can not know whether the displayed car position is reliable or not. 1his makes the navigation system unreliable to the user.